1. Field of the Invention
The present invention relates to apparatus and methods for precision assembly of photoreceptor drums or the like, particularly drums comprised of thin-walled hollow tubes. In general, the present invention further relates to the concentric, distortion-free mounting of an end piece within an open end of a thin-walled hollow tube.
2. Description of the Prior Art
Reprographic devices, such as photocopiers and laser printers, commonly utilize electrostatic principles to reproduce printed materials. In operation, the image to be printed is generated onto a substrate, such as a rotatable photoreceptor drum, by an electrographic process. Toner material is then applied to the substrate to create the image in visible form, which then is transferred and fixed to paper or similar base material.
Recent advancements in reprographic techniques have resulted in greatly enhanced imaging definition. However, these results are dependent upon the high dimensional accuracy and interaction of the various components which make up the reprographic device. In particular, even minute variations in the rotational alignment or surface evenness of the photoreceptor drum can prove detrimental to the image generation process, making efforts toward improving image resolution futile As such, the dimensional accuracy of the photoreceptor drum is critical to the achievement of the desired imaging definition. The primary indicator of dimensional accuracy of the photoreceptor drum is referred to as Total Indirect Run-out (TIR), which is a measure of the trueness of the drum surface and the centricity of the rotational axis.
To date, a photoreceptor drum or the like, is typically assembled using a hollow, open-ended tube and two end bells; each end bell being generally friction-fitted and glued into a corresponding end of the hollow tube. Each end bell includes a central aperture to receive a central axial shaft for rotatably supporting the assembled photoreceptor drum. As such, the hollow tube and end bells must be precision manufactured within a close dimensional tolerance to enable a snug frictional engagement therebetween.
Two alternative mounting configurations of the hollow tube and end bells are conventionally employed in the assembly of photoreceptor drums. In one configuration, as illustratively shown in FIG. 1, a chamfered mounting surface is included along the inside edge of each open end of the hollow tube to snugly receive an end bell therein. As such, the precision manufacture of the hollow tube and end bells includes cutting the tube to the proper length; machining a mounting surface into each open end of the tube truing the exterior surface of the tube on a lathe; and machining the end bells to be concentrically received by corresponding mounting surfaces.
In order to eliminate the expensive process of machining a mounting surface along the inside edge of the hollow tube, the alternative mounting configuration locates the mounting surface along the peripheral edge of the end bell. That is, the end bell of the alternative conventional configuration has a peripheral edge sized to frictionally engage the inside surface of the hollow tube, and an outwardly extending lip having a diameter greater than the inside diameter of the hollow tube. As the end bell of the second configuration is inserted into an open end of the hollow tube, the peripheral edge of the end bell frictionally engages the inner surface of the hollow tube until the lip prevents further insertion.
In either configuration, however, the hollow tube must be constrained during a majority of the manufacturing process. Thus, distortion of the manufactured tube inherently occurs when the constraints are removed and the tube returns to its unstressed, relaxed condition. Likewise, in either configuration, further distortion of the exterior surface of the hollow tube occurs upon insertion of the end bells into the mounting surfaces, due to the snug frictional engagement and tolerance stack-up between the end bells and the mounting surface of the hollow tube, as seen in FIG. 1. Of course, concerns over dimensional inaccuracies are not limited to the distortion of the substrate surface. Tolerance stack-up between the end bells and the hollow drum inherently result in eccentric alignment of the end bells and non-uniform axial distances between the corresponding end bells.
An additional consideration in the manufacture of photoreceptor drums is cost of production. The precision machining processes typically required for the hollow drum and end bells add extraordinary expenses to the overall assembly cost of the photoreceptor drum. Likewise, while thin-walled hollow tubes are preferred for economic reasons, the first conventional method of assembly discussed above prohibits their use since the thin-walled members cannot adequately be machined to include a mounting surface for the end bells.